NMR in biomedicine
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The goal of this study was to validate metabolite quantification at short TE, with particular focus on how to best account for the macromolecular signal contribution. A robust, short-TE PRESS protocol is presented, which allows reliable quantification, in vivo, of metabolite signals at 3 T in human brain. Water suppression was adapted to the experimental conditions at 3 T. ⋯ When initial signal truncation was used, glucose was removed from this set. The short TE (10-11 ms) facilitated quantification of glutamate. The reliable quantification of N-acetylaspartyl glutamate at 3 T proved very difficult.
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Changes in breathing change the concentration of oxygen and carbon dioxide in arterial blood resulting in changes in cerebral blood flow (CBF). This mechanism can be described by the cerebral vascular response (CVR), which has been shown to be altered in different physiological and pathophysiological states. CBF maps of grey matter (GM) were determined with a pulsed arterial spin labelling technique at 3 T in a group of 19 subjects under baseline conditions, hypoxia, and hypercapnia. ⋯ Maps of CVR to hypercapnia showed less intra-subject variation. Per 1 mm Hg increase in partial pressure of end-tidal carbon dioxide, there was a statistically significant 5.8 +/- 0.9% increase in GM-CBF, all subjects showing an overall positive CVR. As the brain is particularly vulnerable to hypoxia, a condition associated with cardiorespiratory diseases, CVR maps may help in the clinic to identify the areas most prone to damage because of a reduced CVR.
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Localized two-dimensional constant-time correlation spectroscopy (CT-COSY) was used to resolve glutamate (Glu), gamma-aminobutyric acid (GABA), and glutamine (Gln) in the human brain at 4.7 T. In this method, three-dimensional localization was achieved using three radio frequency pulses of the CT-COSY module for slice selection. As this sequence could decouple JHH along the F1 direction, peak resolution of metabolites was improved even on a magnitude-mode display. ⋯ Tct was 110 ms; the total scan time was 30 min. Diagonal peaks of Glu C4H, GABA C2H, and Gln C4H were also resolved on the spectrum processed with Gaussian windows. These results show that the localized two-dimensional CT-COSY method featuring 1H decoupling along the F1 direction could resolve Glu, GABA, and Gln signals in the human brain.
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Knowledge of proton T2 relaxation time of metabolites is essential for proper quantitation of metabolite concentrations in localized proton spectroscopy, especially at moderate to long TEs. Although the T2 relaxation time of singlets, such as that of creatine and N-acetylaspartate, has been characterized in several studies, similar information is lacking from coupled spin resonances of cerebral metabolites. In this study, the T2 relaxation time of coupled spin resonances and singlet resonances of cerebral metabolites was measured in rat brain in vivo at 9.4 T. ⋯ Data analysis was performed in the frequency domain with the LCModel software using simulated TE-specific basis sets. The T2 relaxation times in compounds showing singlet resonances were 113 +/- 3 ms (total creatine), 178 +/- 29 ms (total choline) and 202 +/- 12 ms (N-acetylaspartate). The T2 values of J-coupled metabolites ranged from 89 +/- 8 ms (glutamate) to 148 +/- 14 ms (myo-inositol).
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The purpose of this work was to develop magnetic resonance elastography (MRE) for the fast and reproducible measurement of spatially averaged viscoelastic constants of living human brain. The technique was based on a phase-sensitive echo planar imaging acquisition. Motion encoding was orthogonal to the image plane and synchronized to intracranial shear vibrations at driving frequencies of 25 and 50 Hz induced by a head-rocker actuator. ⋯ The observed modulus dispersion indicates a limited applicability of Voigt's model to explain viscoelastic behavior of brain parenchyma within the applied frequency range. The narrow distribution of data within small confidence intervals demonstrates excellent reproducibility of the experimental protocol. The results are necessary as reference data for future comparisons between healthy and pathological human brain viscoelastic data.